Networks connecting a plurality of terminals (nodes) such as computers may have various topologies such as bus, star, ring and mesh topologies. An example of a bus network is Ethernet (registered trademark). Ethernet has a physical star topology but has a logical bus topology, in which each node can freely transmit a signal (data). On the other hand, a signal collision may occur in Ethernet, and in such a case, delivery of the signal is delayed (namely, latency is increased).
Like Ethernet, ring-type networks (hereinafter, may be simply referred to as ring networks) often are wired in a physical star topology in which each network node is connected to a central hub. In ring networks, collision of data on the network does not occur, and the data transmitted onto the network is delivered to the destination node by being relayed from node to node. The final recipient of the transmission data may be the destination node or the source node (namely, there are two communication methods). In the communication method in which the transmission data is to be returned to the source node, the source node can confirm that the communication has been performed correctly by receiving the transmission data, and can know that there is a failure in the network when the data is not returned.
In the ring network, irrespective of which of the communication methods is used, if there is a failure in one communication node, it affects the entirety of the network system and may lead to a system failure. In addition, it is difficult to locate the part where the failure has occurred. It may be conceived to provide bypass switches such that when a failure occurs at a certain communication node, a corresponding bypass switch is operated to cut off the communication node where the failure has occurred. However, if a communication failure (such as a break of the communication line) occurs at a part near and downstream of one of the communication nodes that functions as an upper controller (upper communication node) for the other, the communication between the upper communication node and the other communication nodes in the ring network is prevented and thus the other communication nodes will become unable to operate properly; namely, the entirety of the network system can be affected significantly.
To minimize the affect on the system caused by a communication error, it is known to duplicate the communication lines in a ring network over the entirety of the network (see JP5635029B, for example).
However, when the communication lines are duplicated over the entirety of the ring network as in the invention described in JP5635029B, a work for routing the communication lines through narrow spaces such as joints of a robot can become difficult because four communication lines (two for communication in one direction and two for communication in the opposite direction) need to be passed through each joint. Further, in the case where duplicated communication lines are passed through a moving part such as a joint of a robot, the two communication lines passed through the same moving part can be broken simultaneously.